The object of the invention is an improved filter intended to be placed in the blood stream, particularly in a section of vein, in order to trap blood clots.
Filters of this type are known and are for example described in U.S. Pat. No. 3,952,747 and in the French Patent Application EN 84 14144 filed on Sept. 14, 1984.
Generally speaking, these filters take the form of a small frustoconical basket which is hooked inside a vein downstream of the section which it is desired to filter; generally this is the vena cava which arrives at the heart. And so, before they enter the heart, it is possible to trap any blood clots which may form and which may lead to the formation of emboli.
One difficulty inherent in this type of operation is that of correctly positioning the filter with its axis substantially in the axis of the vein into which it is to be hooked. To facilitate this fitting, the legs of the filter are normally provided with hooks. Normally, in order to insert the filter into the vein, it is pushed in by means of a tube which traverses the vein and the diameter of which is less than that of the vein. When the filter arrives at the end of the tube, it is therefore left in the vein and expansion of its hook-shaped legs provides for anchoring of the filter. The evidence shows that being left in place in this way is, in practice, extremely difficult to monitor. In fact there is scarcely any chance of the basket-like filter occupying, inside the vein, what is the most favourable position, with its axis substantially parallel with the axis of the vein.